The plasma membrane plays a critical role in determining how cells react to external stimuli. Since the plasma membrane has been observed to have altered physiological responses and an altered protein composition in neoplastically transformed cells, it is important to further understand the structure of this organelle. Most of the previous studies on plasma membrane proteins have looked for qualitative or quantitative differences between transformed and untransformed cells. Since many of the protein species are the same in both transformed and normal cells, there is a strong indication that their special organization may be perturbed during transformation. The objectives of this proposal are: (1) to detect which proteins are similar in membrane preparations from a variety of transformed and untransformed cells in culture, (2) to further develop methods for the characterization of the spacial location and interactions between membrane proteins and to compare these organizational features in transformed and untransformed cells. The methods include the use of inside-out membrane preparations which can be selectively labeled or marked on their inside, cytoplasmic, or external surface. The selective interaction of nonpenetrating polymers, the selective enzymatic labeling of the internal or external surface and the selective interaction of monoclonal antibodies, directed against defined externally located cell surface antigens, will be used to study the interactions of membrane proteins. This type of approach has been used to characterize less complex biological structures; however, the technology has improved to the level where the molecular analysis of plasma membrane structure can be initiated. These studies should provide basic insight into the structural organization and alterations in structure which can occur in cells and a better basis for understanding of cellular physiology.